The protection and confidential retention of data in a chipcard constitutes one of the principle advantages compared with other forms of data carriers such as magnetic stripe cards or diskettes. For this reason, a form of chip hardware tailored to this purpose and various cryptographic procedures are known.
Cryptographic procedures include symmetrical and asymmetrical procedures. In the case of a symmetric cryptographic procedure, there exists just one key which is used both for encoding and decoding (sometimes called encrypting and decrypting) the data which can be exchanged with the chipcard. This key must be kept secret as anyone who knows this key can also read information contained in encoded data. This gives rise to the problem of how this key can be exchanged between the communicating partners. It is not possible to pass on the key directly over public networks because then the key would no longer be a secret.
This problem is partially resolved by the assistance of asymmetric cryptographic procedures. In this situation there is a key V for encoding and a key E for decoding. The particular point here is that only one of the two keys has to be kept secret. The key V is known to the general public while the key E is secret. If the sender wishes to send a secret message to a receiving party, he uses the publicly-known key V to encode the information. When the receiving party receives the encoded information, he can decode it with the aid of secret Key E. The reverse situation is also possible where the Key V is secret and the Key E is known to the general public.
The asymmetric cryptographic procedures solve the problem of exchanging the keys. However, a new problem then arises. The authenticity of the publicly-known key must be checked. This takes place by the publicly-known key being certified by a trustworthy authority. To this end, a certificate is produced which demonstrates the following component parts:
a publicly-known key; PA1 the name of the owner of the publicly-known key; PA1 the applications/application areas for which this publicly-known key may be used; and PA1 a digital signature of the trustworthy authority.
From an information-technology point of view, the digital signature amounts to a kind of cryptographic check-sum of the other components of the certificate similar to a MAC (Message Authentication Code) calculated through a prescribed data string. The trustworthy authority creates the digital signature to allow others to confirm that the elements of data (components) in the certificate belong to one another.
There is a standard for the construction and format of a certificate, namely X.509. This Standard arose in association with large data banks and therefore presupposes access to computers with high performance capacities. The evaluation of an X.509 Certificate with the aid of the processor of a chipcard is not possible.
Therefore, in the use of asymmetric cryptographic procedures with chipcards, the chipcard only serves to retain one of the cryptographic keys. Authorization for the use of this key with the asymmetric cryptographic procedure is calculated outside the chipcard by using a computer with a larger computing capacity.